The present invention is related to an oversampled analog to digital converter which is particularly useful in electronic circuit breaker devices. More particularly, the present invention is related to delta-sigma (or .DELTA.-.SIGMA.) modulator devices with provision for input level shifting.
In an electronic circuit breaker, current transformers are typically employed to generate a voltage that is linearly related to the current level flowing through the bus bars in a multi-phase system. This voltage is produced by supplying the secondary current from the current transformers to "burden resistors" at the input pins of an electronic circuit breaker chip. In this fashion, a signal is provided to the circuit breaker chip which is indicative of the current level in a multi-phase power system. However, this same secondary current from the current transformers is also used to generate the power supply to the circuit breaker chip which includes analog to digital converter circuitry. The power is supplied by the use of conventional power supply arrangements comprising diodes arranged in a full wave rectifier configuration, zener diodes for clamping, and capacitive and resistive filter combinations for smoothing. Such power supply arrangements are conventional and well known. However, the utilization of secondary current from the current transformers to supply both signal and power levels to a circuit breaker chip poses significant problems. For example, if the power supply voltage is between 0 and V volts, the input signal level will be between 0 and -V volts. Thus, the problem posed is how to digitize a signal which is outside the range of the reference voltage of the analog to digital converter.
Even more particularly, it is noted that it is desirable to employ delta-sigma analog to digital conversion methods and systems because these are particularly applicable to power system applications since they exhibit high accuracy and a wide dynamic range. Such advantages appear to be appreciated, for example, in U.S. Pat. No. 4,542,354 issued Sept. 17, 1985 to Robinton et al. and in U.S. Pat. No. 4,573,037 issued Feb. 25, 1986, also to Robinton et al. However, the teachings of these patents are not directed to the problem of out-of-supply-range input signals. Nonetheless, it is noted therein that delta-sigma modulation methods are known in the art.
It would appear that there are several ways to solve the out-of-supply-range problem such as the use of a separate power supply also possibly entailing the utilization of more than one set of current transformers. However, most of the alternate methods would appear to employ, out of necessity, off-chip components or extra circuitry on the chip that makes such systems difficult to fabricate economically. Accordingly, it is seen that it is desirable to be able to solve the out-of-supply-range problem in a simple, economical fashion employing on-chip components.